Whiskers and Wiles by Merry Farmer

Whoever marries last will inherit a cursed castle.
Lord Waldorf Godwin has only loved once in his life, and that ended in ruin…
Since his long-ago affair with Lady Katherine Balmor, Waldorf has sworn off love altogether and focused on his duty toward his country instead. His entire country, since Waldorf is firmly on the side of uniting Britannia. He has been charged with one of the most important undercover missions the New Heptarchy has ever seen, but he cannot accomplish the mission alone.

Instead, he is asked to form a team with his arch nemesis, Kat…
Kat has remained furious with Waldorf for almost twenty years after the way he callously tossed her aside. If only her heart would agree with her head when it comes to Waldorf. And now, with the fate of Britannia entrusted to the two of them, her heart seems determined to ruin everything for everyone in Britannia.

When she and Waldorf are forced to pretend to be betrothed for the sake of the mission, anything could happen. And when the very man from the past who came between her and Waldorf shows up again to wreak more havoc, they could be torn apart once more if they cannot learn to let the past go and trust each other.